Bcl-2 Prevents Apoptotic Mitochondrial Dysfunction by Regulating Proton Flux

We and others have recently shown that loss of the mitochondrial membrane potential (Δ ψ ) precedes apoptosis and chemical-hypoxia-induced necrosis and is prevented by Bcl-2. In this report, we examine the biochemical mechanism used by Bcl-2 to prevent Δ ψ loss, as determined with mitochondria isolated from a cell line overexpressing human Bcl-2 or from livers of Bcl-2 transgenic mice. Although Bcl-2 had no effect on the respiration rate of isolated mitochondria, it prevented both Δ ψ loss and the permeability transition (PT) induced by various reagents, including Ca2+, H2O2, and tert-butyl hydroperoxide. Even under conditions that did not allow PT, Bcl-2 maintained Δ ψ , suggesting that the functional target of Bcl-2 is regulation of Δ ψ but not PT. Bcl-2 also maintained Δ ψ in the presence of the protonophore SF6847, which induces proton influx, suggesting that Bcl-2 regulates ion transport to maintain Δ ψ . Although treatment with SF6847 in the absence of Ca2+caused massive H+influx in control mitochondria, the presence of Bcl-2 induced H+efflux after transient H+influx. In this case, Bcl-2 did not enhance K+efflux. Furthermore, Bcl-2 enhanced H+efflux but not K+flux after treatment of mitochondria with Ca2+or tert-butyl hydroperoxide. These results suggest that Bcl-2 maintains Δ ψ by enhancing H+efflux in the presence of Δ ψ -loss-inducing stimuli.